The present invention relates to an antenna for small mobile devices with an antenna rod retractable in an antenna cover, which rod can be in an extendible position, when a terminal operates in an active mode, as well as in a retractable position, when the terminal operates in a passive mode.
There are several and high demands raised on antennas of mobile stations in a modern mobile telephone system. Except for demands on cost efficiency there are demands on long operating time of a mobile station between the recharging of batteries, which means that the electrical energy consumed must be kept low. The energy consumption is influenced by i.a. the efficiency of the antenna.
To meet the requirements from today users, a mobile phone has to be light and compact so that for example it can be stored in an inside pocket in a jacket or carried in a belt by means of a belt-clip. Further, mobile phones are used in different environments and by many different kinds of users, requiring that the telephone has good performance and at the same time is robust and easy to handle.
A mobile station in a modern telecommunication system such as GSM (Global System for Mobile Communications) operates essentially in two modes, an "active" and a "passive" mode. The mobile station is in its active mode during an ongoing call, when the communication is intensive between the mobile station and the present base station, and in the passive mode, the so called stand-by mode or readiness mode, when the mobile station does not serve any ongoing call but some communication with the base station still occurs in regular intervals.
There are several kinds of prior art antennas for mobile stations, such as rod antennas of half wave or quarter wavetype, dipol antennas, coil antennas, helix antennas etc. Different types of antennas have different suitabilities in view of the above described problem perspective. Notwithstanding that the helix antenna does not show as good antenna efficiency as a half wave rod antenna, it is often used because due to its coil shape it can be made compact and durable.
From considerations of space it is further common that mobile phones are provided with retractable antennas, usually completed with separate antennas arranged on the mobile phones for use in readiness state, because a rod antenna fully retracted in a metal casing of an apparatus is isolated from the environment in the meaning of signaling.
EP-A-0 736 925 describes an antenna with dual functions and a mobile phone provided with such an antenna. This antenna has two parts and can operate in two modes, i.e. in an active mode, when the antenna is extended, and a passive mode, when the main part of the antenna is retracted in the casing of the mobile phone.
U.S. Pat. No. 5,204,687 also relates to an antenna which can operate in two modes. The antenna of U.S. Pat. No. 5,204,687 is formed of two conductive elements arranged in series in an elongated antenna structure without galvanic contact with each other. The upper part of the antenna consists of a helix element, and the lower part of the antenna consists of a rod. When the antenna is in its active mode, i.e. the fully extended position, the lower part of the antenna, the rod, operates as an antenna. When the antenna is in its passive retracted position the upper helix element operates as an antenna.
WO92/16980 also describes an antenna, which has dual functions and consists of an upper helix part and a lower rod part. Contrary to the U.S. Pat. No. 5,204,687 antenna the two parts of the antenna co-operates when they are electrically attached to each other. The object of the antenna according WO92/16980 is to dimension the rod part and the cover, in which the rod is located in its retracted position, so that when the antenna operates in the passive mode the rod part constitutes a very high impedance, thereby not influencing the operation of the helix part. An incorrect dimensioned rod part can in its retracted position result in undesired reflections or unnecessary attenuation of the signal.
The object of the invention described in EP-A-0 736 925 is to obtain an antenna structure operating in a suitable way both in retracted position and in extended position and being a small size preferably in mobile telephone systems. Also, it should be easy to produce and be suitable for serial production of mobile stations.
According to what is mentioned above this antenna also consists of two parts electrically attached to each other. The characterizing feature of this antenna is that the length of the lower part of the antenna is essentially less than a quarter of a wave length, at which the antenna is intended to operate, and that the lower part of the antenna in the extended position together with the matching circuit is to be matched to the impedance of the upper part of the antenna, so that it corresponds to the impedance of the antenna port. Thus, the lower part of the antenna is not to be hidden for the signal, which is the case according to WO92/16980. Consequently, the electrical length of the lower part may be considerably less than a quarter of a wave length.
A problem with prior art antennas having a rod portion and a helix portion is the difficulty to mechanically design the antenna so that the requirements of good performance and a compact shape is fulfilled. When the antenna is extended, in many cases the helix section has to be disconnected or compressed in order not to influence the total performance of the antenna. It is also difficult to incorporate the long rod within the telephone if an antenna having an electrical length of a half wave length in extended position is prefered.